Electric blasting initiator



July 13, 1937. BURROWS 2,086,530

ELECTRIC BLASTING INITIATOR Filed 001',- 30, 1935 Law/on AfiurrowsINVENTOR.

ATTORNEY Patented July 13,, 19375 UNITED STATES PATENT OFFIQ ELECTRICBLASTING Application October 30, 1935, Serial No. 47,382

17 Claims.

The present invention relates to electric blasting initiators generally,and more particularly to an electric blasting cap without appreciabletime lag and substantially free from susceptibility to staticelectricity.

Electric blasting caps of all types may be classified as relatively fastand relatively "slow caps on the basis of the time required for the capto detonate after the firing current has been applied. Such aclassification is made to distinguish between blasting caps which aresuitable for ordinary blasting operations and those which are adaptabletorather special uses, as, for example, in seismographic exploration formineral or other deposits where the necessary record of the exactinstant of detonation is most conveniently made by recording the instantof rupture of the bridge wire. Heretofore, however, this distinctionbetween fast and slow caps has been rather qualitative in nature, sinceall the available electric blasting caps possessed an inherent,measurable, and significant time lag or interval between the instant thefiring current was applied and the instant the blasting caps detonated.V

The time lag above referred to is the overall lag resulting from twoseparate factors, the first of which is the interval of time between theapplication of the firing current and the breaking m or fusion of thehigh resistance bridge wire. This interval, which we may term the bridgewire lag is generally dependent upon the firing current used. For manypractical purposes, however, the bridge wire lag is not significant,since in seismographic explorations and other types of Work requiring a"fast cap, the instant the bridge wire breaks or fuses may beconveniently assumed to be the instant of detonation.

A far more serious factor affecting the overall time lag, and one whichis of great significance in the special uses above referred to, is theinterval of time between the breaking of the bridge wire and the actualdetonation-of the cap. This lag, which we shall hereinafter refer to asthe "induction period, depends to a certain extent on the amperage ofthe firing current. At a given amperage, however, the induction period"is a definite characteristic property of the ignition compositionemployed. Thus certain. materials have an inherently long inductionperiod, whereas in others this period is not measurable.

Although a number of materials of the latter for it has been found thatignition compounds which have a negligible or immeasurable inductionperiod are very susceptible to static electricity; that is to say,electrostatic charges of low voltage, which are well within the range ofthat 5 which may be accumulated by a man of ordinary electricalcapacity, are capable of igniting these fast" ignition compounds andthereby initiating the blasting cap. While, in general, it appears thatall ignition compositions can be fired by an electrostatic discharge,this susceptibility to static electricity increases very markedly as theinduction period approaches zero. These rapid ignition compounds cannot, therefore, be employed safely in electric blasting caps unless theelectrostatic susceptibility of the finished blasting cap issubstantially reduced or eliminated by means which are certain in effectand sufliciently rugged in construction to withstand mechanical handlingin the field. Such means are disclosed and claimed generally inco-pending application, Serial No, 47,385 filed October 30, 1935.

The object of the present invention is a new and improved electricblasting cap which is substantially free from susceptibility toelectrostatic charges. A further object is an electric blasting capwithout an appreciable induction period", and substantially free fromsusceptibility to static electricity. A still further object is anelectric blasting cap containing an improved ignition compositionpossessing very rapid ignition characteristics, the electrostaticsusceptibility of said cap being substantially eliminated by means whichare certain in efiect and sufiiciently rugged in construction towithstand mechanical handling in the field. Additional objects will beapparent as the invention is hereinafter more fully described.

I have found that these objects may be accomplished by providing anelectric blasting initiator containing a lead salt of a nitrophencl witha means for substantially reducing the electrostatic susceptibility ofsaid initiator. This means may comprise any of the embodiments of theinvention disclosed and claimed generally in co-pending applicationSerial No. 47,385 filed October 30, 1935.

The lead salts of various nitrophenols may be employed in accordancewith my invention. Thus I may use the lead salts of variouspolynitrophenols, as for example, the lead salts of thepolynitro-monohydric phenols such as picric acid, picramic acid,dinitrosalicylic acid and the like; or the lead salts of thepolynitro-dihydric phenols such as trinitroresorcinol and the like; orthe lead salts of the polynitro-polyhydric phenols such astrinitrophloroglucinol and the like. I prefer, however, to employ thelead salts of trinitrophenol, commonly known as lead picrate; and thelead salts of trin'itroresorcinol, commonly known as lead styphnate.Either the normal or the basic lead salts may be used as desired.

These lead salts of nitrophenols may be employed, if desired, as theignition composition in all types of electric blasting initiators,including electric squibs, electric blasting caps, and delay electricblasting caps, whether of the bridge plug, the concave plug, or thematch-head type. In order to produce an electric blasting initiator,however, which is substantially free from susceptibility to staticelectricity, it is necessary to provide the firing circuit of theinitiator with one of the embodiments of the invention disclosed in theco-pending application above referred to. 7

Although my invention is applicable to all types of electric blastinginitiators, it is particularly applicable to electric blasting caps,since the lead salts of nitrophenols are more especially useful in thistype of initiator. The resulting detonators according to my inventionhave the advan tage not only of being substantially free fromsusceptibility to static but also of being veryrapid, withoutappreciable time lag. The invention therefore makes it possible toproduce a safe, reliable electric blasting cap, suitable for certainspecial uses where the speed of detonation is an importantconsideration.

My invention comprises an electric blasting initiator containing anignition composition comprising a lead salt of a nitrophenol, forexample lead styphnate or lead picrate. The firing circuit of theinitiator is provided with a means whereby the susceptibility of theinitiator to static electricity is substantially eliminated or reduced.The means provided cause the static charge to pass harmlessly to theshell wall at a point outside the locus of the ignition composition, andmay be formed by two general types of procedure, as more fully describedin the co-pending application, Serial No. 47,385 filed October 30, 1935.In the first place, the static electricity may be caused to pass to theshell wall by forming an actual, physicalelectrically-conducting.connection between the shell wall and one of theleg wires at a point outside the locus of the ignition composition. Inthe second place, the static electricity may be caused to pass to theshell wall by forming a spark gap or are between the shell wall and oneof the leg wires at a point outside the locus of theelectrostatic-susceptible ignition composition, such that the resistanceacross the are provided is substantially less than that of the arcformed by the firing circuit and the shell wall at the locus of theignition composition. In either case, the static electricity will passharmlessly to the shell wall, where it will be effectively grounded.

In order to describe my invention more clearly, I shall refer to theattached drawing, which represents several embodiments thereof. It is tobe understood, however, that this is done solely by way of illustration,and is not to be regarded as a limitation upon the scope of my inventionwhich has many important applications other than those hereinparticularly illustrated.

Referring generally to the drawing, Figures 1, 2, and 3 representrespectively vertical sections of a bridge plug type, a concave plugtype, and a match-head type of electric blasting cap, in

accordance with my invention. Similar characters are used throughout theseveral views and in the accompanying description to indicatecorresponding parts.

Referring now particularly to Figure 1, the blasting cap illustratedcomprises a capsule or shell I of a suitable electrically-conductingmaterial, such as a metal, as for example copper, aluminum, and thelike. In the base of the shell I is placed a base charge 2 consisting of80% mercury fulminate and 20% potassium chlorate, and an ignitioncomposition t comprising a lead salt of styphnic acid, as for examplethe red crystalline modification of basic lead styphnate. The lattersurrounds the bridge wire 8, attached to the ends of the leg wires 6 andI. These wires are held in fixed space relationship by means of thebridge plug 5. The cap is closed by means of the waterproofingcomposition 9 and the sulfur seal It. The leg wire I is provided with anelbow II which forms with the shell wall I an arc, the resistance ofwhich is substantially less than that of the arc formed by the shellwall I and the firing circuit at the locus of the ignition compositiont. If desired, the elbow may contact the shell wall and form anelectrical connection therewith.

In the electric blasting cap as illustrated in Figure 1, charges ofstatic electricity which may be inadvertently applied to the shell wallI, or the leg wires 6 and I, will be harmlessly discharged across thearc formed between the elbow I I and the shell wall I without firing theignition composition t and detonating the blasting cap. The electricblasting cap in accordance with my invention is therefore substantiallyfree from susceptibility to static electricity. It is also apparentthat, since the means provided is effectively sealed within the blastingcap, it can not be subsequently impaired by handling in the field.

Referring now to Figure 2, the base charge 2 consists of tetryl, and thepriming charge 3 of lead azide. The bridge wire 8, connected between theends of the leg wires 6 and I is surrounded by an ignition compositioncomprising a lead salt of picric acid, as for example normal leadpicrate gelatinized with nitrostarch and applied preferably in an amylacetate suspension. The leg wires are held in fixed space relationshipby the concave plug I3, and the cap is sealed by the waterproofcomposition 9 and the sulfur seal I0. Aflixed to one of the wires 1 bysuitable means, as for example by solder, is a short length of wire Itwhich forms an electrically-conducting connection between the shell wallI and the leg wire I. By this means, the blasting cap will beeffectively protected from inadvertent firing by static electricity,since the charge will pass to the shell wall I through the connection M,and thence to the ground, without affecting the ignition composition i.

In Figure 3, the match-head type of electric blasting cap illustratedcomprises a detonating base charge 2 of tetryl, a priming charge 3 ofhexanitromannite and a match-head flash or ignition composition 6comprising a lead salt of styphnic acid, as for example, the yellowvariety of basic lead styphnate. Disposed within the paper cylinder I8is the match-head, which comprises two metal plates or foils separatedby a strip of insulating material I5. One leg wire is connected to eachplate, and the bridge wire 8 forms the connection between the foils. Inthe diagram, only the forward plate It is represented.

the ignition composition 4.

, somewhat longer.

The blasting capis closed with the waterproof composition 9 and thesulfur seal III.

The ignition composition 4 may comprise lead styphnate gelatinized withany suitable gelatinizing agent, but I prefer to employ a compositioncomprising approximately 95% lead styphnate and 5% nitrostarch appliedin an amyl acetate suspension, since the resulting composition has beenfound to be very rapid in ignition.

. In Figure 3, the plates or foils of the matchhead, as for example theforward foil l4, are provided with long handles or tabs l6 and ratherbroad shoulders II which extend substantially to the paper tube I 8,which is in contact with the shell wall I. The shell wall I and theshoulders ll will therefore form a low-resistance are or spark gap whichis well outside the locus of The static charge will therefore jumpacross this are without affecting the ignition composition 4. It is ofcourse essential that the ignitioncomposition 4 should cover only thelower part of tab iii of the matchhead, and should not be disposedbetween the shoulders I! and the shell wall I. In shape, the match-headshown is somewhat different from the conventional design in that theshoulders I! are appreciably wider and the tab or handle l6 This change,though apparently slight mechanically, is nevertheless of greatsignificance from the point of view of static susceptibility, for itsubstantially prevents the shoulders I1 from being inadvertently coveredwith the ignition composition 4, which condition I have found willrender the finished cap so sensitive to static that charges of lowvoltage, well within the range of that which a man of average capacitycan accumulate, will invariably fire the ignition composition anddetonate the cap.

As an additional precaution against static, the electric blasting cap asillustrated in Figure 3 is provided with a plug ofelectrically-conducting material I9, which engages the leg. wire 6 andthe shell wall I in electrically-conducting relationship. The plug I9 isseparated from the leg wire 1 by the insulation 20. If desired, the plugI9 may consist of aloW-melting metal or alloy, as for example Babbittmetal, which is poured into the cap about the dielectric material 20surrounding the leg wire 1 The plug l9 may also consist of compressedgraphite or other electricallyconducting material, if desired.

-I have illustrated my invention with particular reference to electricblasting .caps as the preferred embodiment thereof. It is apparent,however, that the invention may also be employed in other electricblasting initiators such as electric squibs, delay electric detonators,and the like, since the electrostatic susceptibility of all initiatorsemploying a lead salt of a nitrophenol in the ignition composition maybe substantially reduced by employing any of the means disclosed hereinor in co-pcnding application, Serial No. 47,385 filed October 30, 1935.Again, the invention is equally applicable to the concave plug type, thebridge plug type, or the match-head type of electric blasting initiator.Furthermore, it may be employed regardless of the base or priming chargeused. Thus, for example, I may employ any of the well-known base chargessuch as tetryl, trinitrotoluene, picric acid, nitromannite,pentaerythritol tetranitrate, and the like. I may use likewise any ofthe well-known priming compositions, as for example lead azide,

diazodinitrophenol, mercury fulminate, nitromannite, and the like.

All of the electric blasting caps illustrated in the drawings anddescribed in the foregoing paragraphs possess a number of very desirableproperties. In the first place, all have an overall time lag of lessthan 6 x 10- seconds, when fired at 12 amperes. In the second place, theinduction period of the caps according to my invention is negligible orimmeasurable when a firing current of 12 amperes is used. Thus, forexample, the cap illustrated in Figure 2 has no measurable inductionperiod, which implies that the bridge wire break is either caused by, oris simultaneous with, the detonation of the cap. Finally, the finishedelectric blasting initiators in accordance with my invention aresubstantially free from susceptibility to static electricity. Thevoltage required to detonate the blasting caps of my invention is farbeyond that which a man of ordinary capacity can accumulate. It isapparent', therefore, that the detonators may be used under normalconditions without danger from electrostatic discharge.

The advantages in the use of my invention are numerous and important. Itprovides a'permanent, simple, easily-manufactured, and' effective meansfor substantially completely eliminating the hazards involved in the useof the lead salts of the nitrophenols. The invention therefore *makes itpossible to produce an electric blasting cap without appreciable timelag and substantially free from susceptibility to static. Heretoforesuch a blasting cap could not be manufactured by any known means.

In the foregoing detailed description of my invention, it is apparentthat many variations may be made without departing from the spirit andscope thereof. I therefore intend to be limited only in accordance withthe following patent claims:

I claim:

1. An electric blasting initiator containing an ignition compositioncomprising a lead salt of a nitrophenol, the firing circuit of saidinitiator being provided with a means wherebxthe susceptibility of theinitiator to static electricity is substantially reduced.

2. An electric blasting cap containing an ignition compositioncomprising a lead salt of a nitrophenol, the firing circuit of saidblasting cap being provided with a means whereby static electricity iscaused to pass harmlessly to the shell .wall of said blasting cap at apoint outside the locus of said lead salt.

3. An electric blasting cap containing an ignition compositioncomprising a lead salt of a polynitrophenol, the firing circuit of saidblasting cap being provided with a means whereby static electricity iscaused to pass harmlessly to the shell wall of said blasting cap at apoint outside the locus of said lead salt.

4. The blasting cap of claim 3, in which said polynitrophenol comprisesa polynitro-monohydric-phenol.

5. The blasting cap of claim 3, in which said polynitrophenol comprisesa polynitro-dihydricphenol.

6. The blasting cap of claim 3, in whic said polynitrophenol comprises apolynitro-polyhydric-phenol.

7. An electric blasting cap of the concave plug type containing anignition composition comprising a lead salt of a 'polynitrophenol, thefiring circuit of said blasting cap being provided with a means wherebystatic electricity is caused to pass harmlessly to the shell wall ofsaid blasting cap at a point outside the locus of said lead salt.

8. The electric blasting cap of claim 7, in which said polynitrophenolcomprises styphnic acid.

9. The electric blasting cap of claim '7, in which said polynitrophenolcomprises picric acid.

10. The electric blasting cap of claim '7, in which said polynitrophenolis gelatinized with nitrostarch.

11. An electric blasting cap of the bridge plug type containing anignition composition comprising a lead salt of a polynitrophenol, thefiring circuit of said blasting cap being provided with a means wherebystatic electricity is caused to pass harmlessly to the shell wall ofsaid blasting cap at a point outside the locus of said lead salt.

12. The electric blasting cap of claim 11, in

which said polynitrophenol comprises styphnic acid.

13. The electric blasting cap of claim 11, in which said polynitrophenolcomprises picric acid.

14. An electric blasting cap of the match-head type provided with anignition composition comprising a lead salt of a poiynitrophenol, thefiring circuit of said blasting cap being provided with a means wherebystatic electricity is caused to pass harmlessly to the shell wall ofsaid blasting cap at a point outside the locus of said lead salt.

15. The electric blasting cap of claim 14, in which said polynitrophenolcomprises styphnic acid.

16. The electric blasting cap of claim 14, in which said poiynitrophenolcomprises picric acid.

17. The electric blasting cap of claim 14, in which said polynitrophenolis geiatinized with nitrostarch.

LAWION A. BURROWS.

